1. Field of the Invention
The present invention relates to a radiation image pickup apparatus and its control method, preferably used for medical diagnosis and industrial non-destructive inspection. In the case of the present invention, electromagnetic waves such as X-rays and γ-rays as well as beams of α and β particles are included in the term “radiation”.
2. Description of the Related Art
Conventionally, an X-ray radiographing system set in a hospital includes a film photographing system for irradiating X-rays to a patient and exposing to the the Xrays that have passed through the patient a film and an image processing system for converting X-rays into electrical signals and performing digital image processing. As one apparatus for realizing the image processing system, there is a radiation image pickup apparatus provided with a scintillator for converting X-rays into visible light and a photoelectric converting apparatus for converting visible light into electrical signals. X-rays that have passed through a patient are applied to a scintillator and the body information on the patient converted into visible light by the scintillator is output from the photoelectric converting apparatus as electrical signals. When the body information on the patient is converted into electrical signals, the electrical signals are digital-converted by an AD converter and X-ray image information for performing recording, display, printing and diagnosis can be handled as digital values.
Recently, a radiation image pickup apparatus is practically used which uses an amorphous silicon semiconductor thin film for a photoelectric converting apparatus.
FIG. 11 is a top view showing a conventional photoelectric converting substrate constituted by using an amorphous silicon semiconductor thin film for materials of an MIS-type photoelectric converting device and a switching device disclosed in U.S. Pat. No. 6,075,256B1 including wirings for connecting the devices. FIG. 12 is a sectional view taken along the line 12-12 in FIG. 11.
A photoelectric converting device 101 and a switching device 102 (amorphous silicon TFT (TFT: Thin Film Transistor); hereafter simply referred to as a “TFT”) are formed on the same substrate 103 and the lower electrode of the photoelectric converting device is shared by a first metallic thin film layer 104 same as the lower electrode (gate electrode) of the TFT and the upper electrode of the photoelectric converting device is shared by a second metallic thin film layer 105 same as the upper electrodes (source electrode and drain electrode) of the TFT. Moreover, first and second metallic thin film layers also share a gate driving wiring 106 and a matrix signal wiring 107 in a photoelectric converting circuit. FIG. 12 shows the total of four pixels of 2×2 pixels as the number of pixels. Hatched portions in FIG. 12 are light receiving faces of a photoelectric converting device. Reference numeral 109 denotes a power source line for supplying a bias to a photoelectric converting device. Moreover, reference numeral 110 denotes a contact hole for connecting a photoelectric converting device with a TFT.
By using the configuration shown in FIG. 11 using amorphous silicon semiconductor as a main material, it is possible to form a photoelectric converting device, switching device, gate driving wiring and matrix signal wiring on the same substrate at the same time and provide a large-area photoelectric conversion circuit unit easily and inexpensively.
Then, device operations of a single photoelectric converting device are described below. FIGS. 13A to 13C are energy band diagrams for explaining device operations of the photoelectric converting device shown in FIGS. 11 and 12. This photoelectric converting device has two types of operation modes such as a refresh mode and a photoelectric converting mode depending on the way of applying a voltage to the first and second metallic thin film layers 104 and 105.
FIGS. 13A and 13B show operations of the refresh mode and operations of the photoelectric converting mode respectively and states in film thickness directions of layers as shown in FIG. 12. M1 denotes a lower electrode (G electrode) formed of the first metallic thin film layer 104 (such as Cr). An amorphous silicon nitride (a-SiNx) layer 111 is an insulating layer for preventing electrons and holes and passage of them, which requires a thickness not having a tunnel effect and is normally set to 500 Å or more. A hydrogeneration amorphous silicon (a-si:H) layer 112 is a photoelectric converting layer formed of an intrinsic semiconductor layer (i layer) not intentionally doped with a dopant. An N+ layer 113 is a single conductivity-type carrier injection preventive layer made of non-singlecrystalline semiconductor such as an N-type a Si:H layer formed to prevent injection of holes into the a-Si:H layer 112. Moreover, M2 denotes an upper electrode (D electrode) formed of the second metallic thin film layer 105 (such as Al).
FIG. 14 is a circuit diagram showing a two-dimensional configuration of a conventional photoelectric converting substrate constituted by using an amorphous silicon semiconductor thin film as the material of a photoelectric converting device and a switching device. However, to simplify the description, the configuration is shown by 9 pixels of 3×3.
In FIG. 14, S1-1 to S3-3 are photoelectric converting devices, T1-1 to T3-3 are switching devices, G1 to G3 are gate wirings for turning on/off the TFTs and M1 to M3 are signal wirings and a Vs line is a wiring for supplying an accumulated bias to the photoelectric converting devices. Electrodes at the black side of the photoelectric converting devices S1-1 to S3-3 are G electrodes and the opposite side is a D electrode. The D electrode is connected with a part of the Vs line. However, to bring light into the D electrode, a thin N+ layer is used as the D electrode. In the case of this conventional example, the photoelectric converting devices S1-1 to S3-3, switching devices T1-1 to T3-3, gate wirings G1 to G3, signal wirings M1 to M3 and Vs line are included in a photoelectric conversion circuit unit 701. The Vs line is biased by a power source Vs. An SR1 is a shift register for applying a driving pulse voltage to the gate wirings G1 to G3 and a voltage Vcom for turning on a TFT is supplied from the outside. Moreover, a control signal VSC is a signal for supplying two types of biases to the Vs line of a photoelectric converting device, that is, the D electrode of the photoelectric converting device. The D electrode becomes Vref(V) when the control signal VSC is set to “Hi” and becomes Vs(V) when the control signal VSC is set to “Lo”. A reading power source Vs(V) and refreshing power source Vref(V) are DC power sources and Vs is set to 9 V and Vref is set to 3 V.
A readout circuit unit 702 amplifies parallel signal outputs of the signal wirings M1 to M3 in the photoelectric conversion circuit unit and series-converts and outputs the signal outputs. RES1 to RES3 are switches for resetting the signal wirings M1 to M3. A1 to A3 are amplifiers for amplifying signals of the signal wirings M1 to M3, CL1 to CL3 are sample holding capacitors for temporarily storing signals amplified by the amplifiers A1 to A3, Sn1 to Sn3 are switches for sample holding, B1 to B3 are buffer amplifiers, Sr1 to Sr3 are switches for series-converting parallel signals, SR2 a shift register for supplying pulses for series conversion to the switches Sr1 to Sr3, Ab is a buffer amplifier for outputting a series-converted signal.
Then, operations of the photoelectric conversing apparatus shown in FIG. 14 are described below. FIG. 15 is a time chart showing operations of the conventional photoelectric converting apparatus shown in FIG. 14.
The control signal VSC supplies two types of biases to the Vs line, that is, D electrodes of the photoelectric converting devices (S1-1 to S3-3). The D electrodes become Vref(V) when the control signal VSC is set to “Hi” and Vs(V) when the control signal VSC is set to “Lo”. The reading power source Vs(V) and refreshing power source Vref(V) are DC power sources.
First, operations in the refresh period are described. All signals of the shift register SR1 are set to “Hi” and the CRES signal of the readout circuit unit 702 is set to “Hi”. Thus, all switching TFTs (T1-1 to T3-3) are turned on, the switching devices RES1 to RES3 in the reading circuit 702 are also turned on and G electrodes of all photoelectric converting devices (S1-1 to S3-3) become the GND potential. Moreover, when the control signal VSC is set to “Hi”, D electrodes of all photoelectric converting devices (S1-1 to S3-3) become a state biased to the refreshing power source Vref(V) (negative potential). Thereby, all photoelectric converting devices (S1-1 to S3-3) become the refresh mode and refreshing is performed.
Then, a photoelectric converting period is described. When the control signal VSC is changed to the state of “Lo”, D electrodes of all photoelectric converting devices (S1-1 to S3-3) become a state biased by the reading power source Vs. Thus, the photoelectric converting devices (S1-1 to S3-3) become the photoelectric converting mode. In this state, all signals of the shift register SR1 are set to “Lo” and the CRES signal of the reading circuit 702 is set to the state of “Lo”. Thereby, all switching TFTs (T1-1 to T3-3) are turned off, the switching devices RES1 to RES3 in the reading circuit 702 are also turned off, G electrodes of the photoelectric converting devices (S1-1 to S3-3) are opened in DC. However, potentials of the photoelectric converting devices (S1-1 to S3-3) are kept because they have capacitive element components as components.
At this point of time, electric charges are not generated because light does not enter the photoelectric converting devices (S1-1 to S3-3). That is, no current flows. In this state, when a light source is turned on like a pulse, light is applied to D electrodes (N+ electrodes) of the photoelectric converting devices (S1-1 to S3-3) and the so-called photoelectric current flows. Though the light source is not illustrated in FIG. 14, a fluorescent lamp, LED or halogen lamp is used in the case of a copying machine. In the case of an X-ray radiographing apparatus, an X-ray source is literally used as a light source. In this case, it is allowed to use a scintillator for converting X-rays into visible light. Moreover, photoelectric current flown by light is stored in photoelectric converting devices (S1-1 to S3-3) as electric charges and kept after a light source is turned off.
Then, a reading period is described. The reading operation is performed from the photoelectric converting device (S1-1 to S3-3) at the first line to photoelectric converting devices (S2-1 to S2-3) at the second line and photoelectric converting devices (S3-1 to S3-3) at the third line, in order.
First, a gate pulse is supplied to the gate wirings G1 of the TFTs (T1-1 to T1-3) of a switching device from the SR1 in order to read the photoelectric converting devices (S1-1 to S1-3) at the first line. In this case, a high-level gate pulse is a voltage V (on) supplied from the outside. Thereby, the TFTs (T1-1 to T1-3) are turned on and signal charges stored in the photoelectric converting devices (S1-1 to S1-3) at the first line are transferred to the signal wirings M1 to M3.
A reading capacitive element is added to the signal wirings M1 to M3 though not illustrated in FIG. 14 and the signal charges are transferred to the reading capacitive elements through the TFTs (T1-1 to T1-3). For example, the reading capacitive element to which the signal wiring M1 is the summation of inter-electrode capacitive elements (Cgs) (three capacitive elements) between gates and sources of the TFTs (T1-1 to T3-1) connected to the signal wiring M1. Moreover, signal charges transferred to the signal wirings M1 to M3 are amplified by amplifiers A1 to A3. Then, by turning on a SMPL signal, the signal is transferred to sample holding capacitive elements CL1 to CL3 to turn off the SMPL signal and the capacitive elements CL1 to CL3 are held.
Then, by applying a pulse from the shift register SR2 to the switches Sr1, Sr2 and Sr3, signals held by the sample holding capacitive elements CL1 to CL3 are output from the amplifier Ab in order of the sample holding capacitive elements CL1, CL2, CL3. As a result, photoelectric conversion signals for one line of the photoelectric converting devices (S1-1 to S1-3) are sequentially output. Read operations of the photoelectric converting devices (S2-1 to S2-3) at the second line and read operations of the photoelectric converging devices (s3-1 to S3-3) at the third line are similarly performed.
When signals of the signal wirings M1 to M3 is sample-held in the sample holding capacitive elements Cl1 to CL3 in accordance with the first-line SMPL signal, the signal wirings M1 to M3 are reset to the GND potential in accordance with a CRES signal and thereafter, a gate pulse can be applied to a gate wiring G2. That is, it is possible to transfer signal charges of the photoelectric converting devices (S2-1 to S2-3) at the second line by the shift register SR1 while performing the series converting operation of the signal at the first line by the shift register SR2.
According to the above operations, it is possible to output signal charges of all the photoelectric converting devices (S1-1 to S3-3) from the first line to the third line.
Operations of the X-ray radiographing apparatus described above are operations for obtaining one static image as it were by performing the refresh operation, applying X-rays and performing the read operation. Moreover, to obtain continuous dynamic images, it is only necessary to operate the time chart shown in FIG. 15 repeatedly, a number of times equal to the number of dynamic images to be obtained.
FIG. 16 shows a two-dimensional circuit configuration of a photoelectric converting apparatus using not MIS-type photoelectric converting device but a PIN-type photoelectric converting device. In FIG. 16, only 9 pixels=3×3 pixels are shown similarly to FIG. 14.
In the case of the PIN-type photoelectric converting device, a P layer is constituted. This is not included in the switching device (TFT) shown in FIG. 11. That is, as shown in FIG. 11, it is impossible to simultaneously constitute a photoelectric converting device and a switching device on the same substrate. Therefore, because a constituting method becomes complex compared to the case in FIG. 11, the manufacturing cost may become high.
However, the PIN-type photoelectric converting device has no insulating layer (injection element layer) differently from the MIS-type photoelectric converting device, electrons and holes can move in both directions. Therefore, it is unnecessary to perform the refresh operation described for the MIS-type photoelectric converting device.
FIG. 17 is a time chart showing operations of the conventional photoelectric converting apparatus shown in FIG. 16. As shown in FIGS. 17 and 15, in the case of the PIN-type photoelectric converting device in FIG. 17, there is no refresh operation. By repeatedly operating the read timing, the PIN-type photoelectric converting device may be advantageous compared to the MIS-type photoelectric converting device in speed when obtaining a dynamic image.
However, particularly in the case of a medial radiation radiographing apparatus, a specification is requested in which a radiographing region is as very large area as 40-cm square in order to radiograph a personal chest. In this case, the capacitive element being parasitic on the signal wirings M1 to M3 ranges between 50 and 200 pF though depending on design even if using either one of the MIS-type photoelectric converting device or the PIN-type photoelectric converting device. These parasitic capacitive elements are the capacitive element between top and bottom of a TFT electrode, capacitive element parasitic at the cross portion between driving wiring and signal wiring and capacitive element parasitic between signal wiring and bias wiring (Vs line) of a photoelectric converting device.
However, when radiographing pixels are arranged at a 200 μm pitch, the pixel capacitive element ranges between 1 and 3 pF. If the capacitive element of a signal wiring is 100 pF and a pixel capacitive element is 2 pF, when performing the transfer operation through a TFT, a signal voltage lowers to 2 pF/(2 pF+100 pF)≡1/50 at the front and rear of the TFT. In this case, because noise components of a rear-stage readout circuit unit to be connected to a signal wiring, for example, the so-called circuit noises such as thermal noises of a resistance and shot noises of a transistor are not zero, there is a problem in that S/N is lowered. This problem occurs when the photoelectric converting device is either the MIS-type or the PIN-type.
Therefore, in this embodiment, an operational amplifier is provided for each signal wiring, and the size of a differential transistor at the initial stage of the operational amplifier is increased for decreasing the circuit noise of the readout circuit unit 702. However, this structure has the problem that the number of operational amplifiers increases and the chip size increases. Moreover, there are problems in that current consumption increases and calorific output increases. Furthermore, problems that a cooling mechanism must be mounted and, thereby, the apparatus becomes more complex are induced.
Furthermore, as one method for solving deterioration of S/N, U.S. Pat. No. 6,600,160B1 discloses a method for inputting a signal potential from a photoelectric converting device to the gate of a TFT and outputting the TFT as a source follower. In this case, because an output signal of the photoelectric converting device is not deteriorated but it is input to a read circuit, it is considered that this is advantageous for S/N.
In this case, however, noises superimposed on a sensor bias wiring, that is, noises by a bias power source are output through the TFT serving as a source follower similarly to the case of signal components. These noises are included in a conventional circuit which does not output noises as the source follower shown in FIG. 14 or FIG. 16. However, because the noises are buried in noises of a rear-stage read circuit, they tend not to become comparatively conspicuous as images.
However, in the case of the apparatus disclosed in U.S. Pat. No. 6,600,160B1, noise components by the bias power source performs scanning in an image pickup circuit unit or sample holding in a reading circuit unit for every line similarly to the case of a signal. Therefore, there is a problem of inducing horizontal-line noises (hereafter referred to as line noises). The line noises have a problem of deteriorating an image quality compared to noises generated at random for every pixel (hereafter referred to as random noises).
Moreover, noises to be superimposed on a sensor bias wiring include noises due to a bias power source and external noises spatially incoming to the bias wiring from the outside. The system disclosed in U.S. Pat. No. 6,600,160B1 is able to read signals of a photoelectric converting device without loss and outputting them to a readout circuit unit by the system includes a problem that it has no resistance against external noises incoming to the photoelectric converting device, particularly the bias wiring.
Furthermore, a PIN-type photodiode is used as the photoelectric converting device disclosed in U.S. Pat. No. 6,600,160B1. Because the PIN-type photodiode does not require the refresh operation necessary for a MIS-type photoelectric converting device, it has less problems that it is difficult to apply the photodiode to dynamic-image photographing related to the refresh operation.
However, because the PIN-type photodiode requires two junctions such as PI junction and IN junction, it has the problem that dark current increases. Particularly, a P layer is a layer peculiar to a photoelectric converting device and it is completely different from the fabrication process of the other TFTs formed on the same substrate. This represents that there is a problem that a laminated structure is formed because it is necessary separately to fabricate a TFT and a photoelectric converting device, and as a result the structure is disadvantageous in yield and cost.
However, when using an MIS-type photoelectric converting device, it is possible to obtain a dynamic image by continuously repeating the read operation as described above. However, by switching the bias power source of the photoelectric converting device, it is necessary to perform the refresh operation and there is a problem that the speed is decreased by the time equivalent to the refresh operation.
Particularly, in the case of a medical image pickup apparatus, the area increases and the number of pixels is inevitably increased. For example, when fabricating an X-ray radiographing apparatus by setting the radiographing region to 40-cm square and the pixel pitch to 200 μm, the number of photoelectric converting devices reaches 4,000,000. To simultaneously refresh these many pixels through a bias wiring as examples shown in FIGS. 14 and 15, it is necessary to apply X-rays by waiting the convergence of voltage fluctuations of GND and power source line of the X-ray radiographing apparatus because the current to be transiently flown at the time of refresh also increases and voltage fluctuations of the GND and power source line increase. That is, a system for simultaneously refresh bias wirings has a problem that it is impossible to achieve a high frame rate as a dynamic image.
Thus, in the case of the prior art for refreshing all photoelectric converting devices once for every operation for reading one frame, dynamic-image photographing is difficult.